462 COSMIC PIIILOSOriLY. L pt. il 



strikingly dissimilar. If the distribution of organisms were 

 miraculously determined in accordance with their fitness to 

 their surrounding conditions, the fauna of South America in 

 latitude 35° ought to resemble the fauna of Australia in the 

 same latitude more closely than it resembles the fauna of 

 South America in latitudes north of 25°. The case is just 

 the reverse. Again there is no appreciable difference 

 between the conditions of existence in the seas east and 

 west of the isthmus of Panama ; and, according to the 

 assumption of the special-creationists, their marine faunas 

 ought to be almost exactly alike. In fact no two marine 

 faunas are more completely distinct. Hardly a fish, mol- 

 lusk, or crustacean is common to the eastern and western 

 shores. This is because the isthmus, though narrow, is im- 

 passable for marine organisms. On the other hand, wherever 

 groups of organisms are not prevented by impassable barriers 

 from spreading over wide tracts of country or of sea, we find 

 distinct but closely-allied species widely spread and living 

 among the most diverse conditions. The inference is obvious 

 that the population of different zoological and botanical areas 

 is due to migration, and not to special creation. Where 

 organisms have a chance to migrate, they migrate, and 

 became adapted, by slight specific changes, to the new cir- 

 cumstances which they encounter. But where there is a 

 barrier between one area and another, there we find complete 

 diversity between the inhabitants of the two areas, although 

 shere is no reason for such diversity, save the impossibility 

 of getting across the barrier. Of like meaning is the fact 

 that batrachians and terrestrial mammals are never found 

 indigenous upon oceanic islands. As Mr. Darwin observes, 

 " the general absence of frogs and toads from oceanic islands 

 cannot be accounted for by their physical conditions ; indeed 

 it seems that islands are peculiarly well fitted for these 

 animals ; for frogs have been introduced into Madeira, the 

 Azores, and Mauritius, and have multiplied so as to be- 



